EVOLUTION AND THE CONCOMITANT DISAPPEARANCE OF HIGH-TC SUPERCONDUCTIVITY WITH CARRIER CONCENTRATION IN THE LA2-XSRXCUO4-DELTA SYSTEM (0.0-LESS-THAN-X-LESS-THAN-1.2) - CROSSOVER FROM A MOTT INSULATOR TO A BAND METAL

Electrical resistivity, magnetic susceptibility, thermogravimetric analysis, and infrared absorption spectra of the compound La2-xSrxCuO4- have been studied for a wide range of Sr concentrations (0.0 x 1.2). The samples annealed at an oxygen pressure of 1 bar were stoichiometric ( =0.0) in the range 0.0<x<0.33. In this range the compounds are characterized by a decrease in the a parameter, an increase in the c parameter, and a maximum in the c/a ratio (at x=0.33) typical of the formation of low-spin Cu3+ ions. In the range 0.15<x<0.33, the compounds show a positive temperature coefficient of resistivity, decrease in the magnitude of the Pauli magnetic susceptibility, infrared oscillator strengths, thermopower S, as well as the Hall coefficient RH. The superconducting transition temperature Tc as well as the percentage Meissner fraction also decrease with x in this range. In particular the stoichiometric x=0.33 composition having a hole concentration of 0.33 holes/Cu shows a minimum in the Pauli magnetic susceptibility and disappearance of all infrared absorption bands. The superconductivity also disappears down to 4.2 K at this composition, even though it is more metallic. These results have been attributed to the occurrence of a transition from a highly correlated narrow-band Mott conductor to a broadband metal at high carrier concentrations. At still higher Sr concentrations (0.33<x<1.0), holes and oxygen vacancies coexist. The decrease in the c/a ratio, increase in resistivity, reappearance of the infrared bands, and the Curie-type magnetic susceptibility observed in the range 0.66 x 1.2 indicate the dominating role of oxygen vacancies which induce disorder and localization. © 1990 The American Physical Society.